Technical Field
The present invention relates to an encoder, and particularly, to an optical encoder.
Related Art
There are now optical linear encoders as one type of devices for measuring an amount of movement. An optical linear encoder has a scale, and a detection head configured to move along the scale. The scale has an absolute pattern for detecting a reference position, and an incremental pattern for detecting an amount of relative movement between the scale and the detection head. The optical linear encoder determines a reference position based on a reference position signal representing a result of detection on the absolute pattern formed on the scale. Then, the optical linear encoder can detect the positional relation between the scale and the detection head, in view of an amount of movement from the reference position.
Also, there are known single-track absolute encoders which are configured by integrating an incremental pattern and an absolute pattern as a single scale track. In such a single-track absolute encoder, incremental tracks (an incremental pattern) and absolute tracks (an absolute pattern) are formed in a stripe pattern, and the absolute tracks are distributed in a pseudo-random manner (Patent Document 1). Therefore, the single-track absolute encoder detects a combination of signals corresponding to the absolute tracks, thereby detecting an absolute position.
Also, there has been proposed a method of measuring an absolute position where there is absolute data (an absolute pattern) formed in a discrete codeword form by removing some light reflecting lines, in an incremental scale (an incremental pattern) configured by light reflecting lines (Patent Document 2).
Further, there has been proposed a measuring scale for determining an absolute value of a reader relative to the scale in two directions (Patent Document 3). In this measuring scale, in order to detecting positions in the two directions, a checkered pattern is formed on the scale, and blocks of the pattern are imaged and decoded. The blocks include information bits defining absolute positions, and this pattern can also be used for measuring an increment.    Patent Document 1: Japanese Patent Application Laid-Open No. 5-71984    Patent Document 2: Japanese Patent Application Laid-Open No. 2004-529344    Patent Document 3: Japanese Patent Application Laid-Open No. 2001-194187
According to the above-described single-track absolute encoder, it is possible to detect the absolute position, and in order to improve the accuracy of the encoder, it is required to decrease the pitch of a scale. However, in all single-track absolute encoders of an optical type, a magnetic type, and an electrostatic capacitance type, even if fine scale pitches are set, it is impossible to read scales due to limitations attributable to their detection principles.